Intel Publishes New Linux P-State Driver

Dirk Brandewie has published a new P-state driver for Intel Core CPUs on Linux. This new cpufreq performance state scaling driver initially is supporting just Sandy Bridge processors but will be expanded to handle other Intel hardware.

Intel's P-State Linux kernel driver is following a similar design to Transmeta's scaling driver. Here's the patch-set description from the kernel mailing list:

This driver implements a scaling driver with an internal governor for Intel Core processors. The driver follows the same model as the Transmeta scaling driver (longrun.c) and implements the setpolicy() instead of target(). Scaling drivers that implement setpolicy() are assmuned to implement internal governors by the cpufreq core. All the logic for selecting the current P state is contained within the driver no external governor is used by the cpufreq core.

At the moment only Intel SandyBridge processors are supported. As testing on SandyBridge+ processors is completed support will be added to the driver.

New sysfs files for controlling P state selection have been added to /sys/devices/system/cpu/intel_pstate/

max_perf_pct: limits the maximum P state that will be requested by the driver stated as a percentage of the avail performance.

min_perf_pct: limits the minimum P state that will be requested by the driver stated as a percentage of the avail performance.

no_turbo: limits the driver to selecting P states below the turbo frequency range.

The units for these for these files are purposely abstract and stated in terms of available performance and not frequency. In idea that frequency can be set to a single frequency is a fiction for Intel Core processors. Even if the scaling driver selects a single P state the actual frequency the processor will run at is selected by the processor.